Specialty materials processing techniques for enhanced resonant frequency hexaferrite materials for antenna applications and other electronic devices

ABSTRACT

Processing techniques for forming a textured hexagonal ferrite materials such as Z-phase barium cobalt ferrite Ba 3 Co 2 Fe 24 O 41  (Co 2 Z) to enhance the resonant frequency and other magnetic properties of the material for high frequency applications are provided. The processing techniques include magnetic texturing by using fine grain particles and sintering the material at a lower temperature than conventional firing temperatures to inhibit reduction of iron. The processing techniques also may include aligning M-phase (BaFe 12 O 19  uniaxial magnetization) with non-magnetic additives in a static magnetic field and reacting with BaO source and CoO to form Z-phase (Ba 3 Me 2 Fe 24 O 42 ). In some implementations, processing techniques includes aligning Co 2 Z phase (planar magnetization) with magnetic texturing occurring in a rotating magnetic field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Application No. 61/420,625 filed on Dec. 7, 2010 andU.S. Provisional Application No. 61/435,608 filed on Jan. 24, 2011. Eachof the application is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to methods of preparing compositionsand materials useful in electronic applications, and in particular,useful in radio frequency (RF) electronics.

2. Description of the Related Art

Magneto-dielectric materials are particularly useful in RF devices suchas antennas, transformers, inductors, and circulators. Recent advancesin magneto-dielectric materials are driven in part by the need tominiaturize high frequency antennas while maintaining desirablebandwidth, impedance, and low dielectric loss. It is also desirable toincrease the upper frequency limit of an antenna, which is largelydetermined by the resonant frequency of the material used.

Hexagonal ferrites such as Z-phase barium cobalt ferrite(Ba₃Co₂Fe₂₄O₄₁), commonly abbreviated as Co₂Z, are magneto-dielectricmaterials often used in high frequency antennas and other RF devices. Toimprove the performance characteristics of Co₂Z and other hexagonalferrites, prior art methods are largely focused on substituting certainchemical elements in Co₂Z with others. For example, one such methodinvolves doping Co₂Z with small amounts of an alkali metal such aspotassium (K), sodium (Na), or rubidium (Rb) to improve the magneticpermeability of the material at high frequencies, which in turnincreases the useable frequency range. However, these chemicalsubstitution solutions are met with moderate success. As such, there isa continuing need to improve the material properties and performancecharacteristics of magneto-dielectric materials such as Co₂Z for RFapplications.

SUMMARY OF THE INVENTION

The compositions, materials, methods of preparation of this disclosureeach have several aspects, no single one of which is solely responsiblefor its desirable attributes. Without limiting the scope of thisinvention, its more prominent features will now be discussed briefly.

Certain embodiments of the invention provide a method of increasing theresonant frequency of hexagonal ferrite materials. In one embodiment,the method comprises forming a fine grain hexagonal ferrite powder in adesired phase and firing the hexagonal powder at a low temperature,preferably lower than standard sintering temperatures for the particularmaterial. In some embodiments, the method further comprises compactingthe hexagonal ferrite powder before firing. In one implementation, thehexagonal powder is fired at a temperature between about 1100° C. to1250° C. In another implementation, the hexagonal ferrite powder has anaverage particle size of less than 1 micron, preferably between about300 nm-600 nm. In yet another implementation, the hexagonal ferritepowder has a surface area of greater than about 6 m²/g, preferablygreater than about 15 m²/g. The resulting material is preferably a finegrained hexagonal ferrite material having a density in the range ofabout 70%-100% of the theoretical density. The processing techniquescause the hexagonal ferrite material to have reduced magnetorestriction,which increases the resonant frequency of the material and, in turn,results in higher frequency values for antenna applications.

The hexagonal ferrite materials can include Z type hexagonal ferritessuch as MI₃MII₂Fe₂₄O₄₁, Y type hexagonal ferrites such asMI₂MII₂Fe₁₂O₂₂, W type hexagonal ferrites such as MIMII₂Fe₁₆O₂₇, U typehexagonal ferrites such as MI₄MII₂Fe₃₆O₆₀, X type hexagonal ferritessuch as MI₂MII₂Fe₂₈O₄₆, and M type hexagonal ferrites such asMIFe₁₂-2xMIIxMIII_(x)O₁₉, wherein MI is barium (Ba) or strontium (Sr),and MII is a divalent metal such as cobalt (Co), zinc (Zn), nickel (Ni),magnesium (Mg), Manganese (Mn), or copper (Cu), MIII is a tetravalentmetal such as titanium (Ti), zirconium (Zr), tin (Sn), germanium (Ge),silicon (Si), cerium (Ce), praseodymium (Pr), or hafnium (Hf).

Advantageously, the preferred embodiments of the invention provide amethod to produce fine grain hexagonal ferrite materials having reducedmagnetorestriction and increased resonant frequency without modifyingthe chemical composition of the hexagonal ferrite. However, in someembodiments, intergrowths between different phases of materials canapply. Small amounts of dopants such as potassium (K), sodium (Na),rubidium (Rb), or calcium (Ca) can also be added to the hexagonalferrite further modify the properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the microstructures of Co₂Z of one embodiment at 500×magnification;

FIG. 2 is a flow chart illustrating a method of forming a hexagonalferrite material according to one preferred embodiment;

FIG. 3 is a flow chart illustrating a method of forming a hexagonalferrite material according to another preferred embodiment;

FIG. 4 is an impedance plot illustrating the results of lower resonancepeaks achieved when the material is prepared without zeta milling andwithout low firing;

FIG. 5 is an impedance plot illustrating the results of lower resonancepeaks achieved when the material is zeta milled and fired at a highertemperature;

FIG. 6 is an impedance plot illustrating the results of higher resonantpeaks achieved using methods of the preferred embodiments to processhexagonal ferrite material;

FIG. 7 schematically shows an example of a circulator incorporating anenhanced resonant frequency Co₂Z material formed in accordance withprocessing techniques of certain embodiments of the present invention;and

FIG. 8 illustrates a telecommunication base station system incorporatinga Co₂Z material formed in accordance with processing techniques ofcertain embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Magnetic materials are preferred in high frequency applications such asantennas, transformers, inductors, circulators, and absorbers because ofcertain favorable material properties. Some of the desirable propertiesafforded by magnetic materials are favorable miniaturizing factors,reduced field concentration, and better impedance match. Hexagonalferrite systems, in particular, are desirable because of their highmagnetic permeability and absorption at microwave (100 MHz-20 GHz)frequencies. Hexagonal ferrite systems include crystal structures thatare generally intergrowths between magnetoplumbite and spinel structurescontaining barium (Ba) or strontium (Sr), a divalent cation such as iron(Fe), cobalt (Co), nickel (Ni) or manganese (Mn) and trivalent Fe. Thehexagonal ferrite may be formed in a variety of different crystalstructures based on the magnetoplumbite cell. These structures includeM-phase (BaFe₁₂O₁₉), W-phase (BaMe₂Fe₁₆O₂₇), Y-phase (Ba₂Me₂Fe₁₂O₂₂) andZ-phase (Ba₃Me₂Fe₂₄O₄₂).

Embodiments of the present invention disclose methods and processingtechniques for improving performance characteristics of hexagonalferrite materials used in high frequency applications. Certain preferredembodiments provide improved methods and processing techniques formanufacturing Z-phase hexagonal ferrite systems Ba₃Co₂Fe₂₄O₄₂ (Co₂Z)that have reduced magnetorestriction, improved resonant frequency, andextended magnetic permeability at higher frequencies. FIG. 1 illustratesthe microstructures of Co₂Z of one embodiment at 500× magnification.

Relative permeability and relative permittivity are propertiesindicative of the performance of a magnetic material in high frequencyapplications. Relative permeability is a measure of the degree ofmagnetization of a material that responds linearly to an appliedmagnetic field relative to that of free species (μ_(r)=μ/μ_(o)).Relative permittivity (∈_(r)) is a relative measure of the electronicpolarizability of a material to the polarizability of free species(∈_(r)=∈/∈_(o)). Generally, permeability (μ′) can be separated into twocomponents: spin rotational X_(sp) which is in response for highfrequency, and domain wall motion X_(dw) which is damped out atmicrowave frequencies. Permeability can be generally represented byμ′=1+X_(dw)+X_(sp).

Unlike spinels, Co₂Z systems typically have a non-cubic unit cell,planar magnetization, and an anisotropic spin-rotation component topermeability. Spin rotation anisotropy is also a consideration inpreparing Co₂Z for high frequency applications. Large anisotropy fields(H_(θ)) are similar to applying an external magnetic field whichincreases resonant frequency, whereas small anisotropy fields (H_(φ))improve permeability. H_(θ) is generally strong in hexagonal ferrites,such as Co₂Z. As such, domain formation out of the basal plane issuppressed and the material becomes self-magnetizing. The relationshipbetween permeability and rotational stiffness can be represented by theformula (μ_(o)−1)/4π=(⅓)(M_(s)/H_(θ) ^(A)+M_(s)/H_(φ) ^(A)). Forisotropic rotational stiffness in connection spinels and c-axis orientedhexagonal ferrites, the relationship can be represented as follows:(μ_(o)−1)/4π=(⅔)(M_(s)/H^(A)). For cases where H_(θ) ^(A) does not equalto H_(φ) ^(A): f_(res) (μ_(o)−1)=4/3ψM_(s[)½ (H_(θ) ^(A)/H_(φ) ^(A))+½(H_(φ) ^(A)/H_(θ) ^(A))]. It is believed that the larger the differencein rotational stiffness, the greater the self magnetization field andthe greater the resonant frequency, which could push the resonantfrequency into the microwave region. Permeability drops quickly aboveresonance frequency.

Certain aspects of the present disclosure provide processing techniquesfor increasing the permeability of Co₂Z at higher frequencies. In oneimplementation, the processing techniques involve methods of magnetictexturing of Co₂Z to result in a textured Co₂Z with improved magneticproperties. In one embodiment, the method of magnetic texturing used informing Co₂Z involves using a reaction sintering method, which includesthe steps of aligning M-phase (BaFe₁₂O₁₉ uniaxial magnetization) withnon-magnetic additives in a static magnetic field and reacting with BaOsource and CoO to form Z-phase (Ba₃Me₂Fe₂₄O₄₂). In another embodiment,the method of magnetic texturing used in forming Co₂O involves using arotating magnetic field method, which includes the steps of aligningCo₂Z phase (planar magnetization) with magnetic texturing occurring in arotating magnetic field. The inventor has found that the degree ofalignment, thus permeability gain, is far superior in a rotatingmagnetic field.

In some embodiments, the processing technique for forming Co₂Z includesmaking Z phase Fe deficient to inhibit reduction of Fe as the inventorbelieves that dielectric and magnetic loss is increased by reduction ofFe (Fe³⁺→Fe²⁺) at high temperatures. The processing technique includesthe step of heat treatment or annealing in oxygen to inhibit reductionof Fe and cause Fe²⁺→Fe³. In other embodiments, the processing techniqueincludes doping the Co₂Z with additives such as potassium and alkalimetals to increase the resonance frequency, and hence increase Q athigher frequency ranges.

In some other embodiments, the processing technique for forming Co₂Zincludes forming fine grain hexagonal ferrite particles. The processinvolves using high energy milling to reduce the particle size. Thefollowing chart shows that in one embodiment, high energy milling isused to produce Co₂Z particle size in the range of 0.2 to 0.9 micronsand surface area of 8-14 m²/g. In this embodiment, the firingtemperature is preferably 1150 to 1250° C.

Grain Size Particle Surface Firing (Intercept Process Size Area Temp.Method) Standard D50 = 1-5  1-3 m²/g 1250-1350° C. 10-30 microns Millingmicrons High Energy D50 = 0.2-0.9 8-14 m²/g 1150-1250° C.  2-15 micronsMilling microns

FIG. 2 illustrates a method 100 of forming a Co₂Z material according toa preferred embodiment. As shown in FIG. 2, appropriate amounts ofprecursor materials—reactants that may provide barium, cobalt, iron, oneor more alkali metals, and oxygen that can form the magneticmaterial—are mixed together in Step 102. In some aspects, at least aportion of the oxygen may be provided in the form of anoxygen-containing compound of barium (Ba), cobalt (Co), iron (Fe), orone or more alkali metals. For example, these elements may be providedin carbonate or oxide forms, or in other oxygen-containing precursorforms known in the art. In one or more aspects, one or more precursormaterials may be provided in a non-oxygen-containing compound, or in apure elemental form. In other aspects, oxygen could be supplied from aseparate compound, such as, for example, H₂O₂ or from gaseous oxygen orair. For example, in one embodiment, BaCO₃, CO₃O₄, and Fe₂O₃ precursorsare mixed in a ratio appropriate for the formation of CO₂Z (for example,about 22 wt. % BaCO₃, about 6 wt. % CO₃O₄, and about 72 wt. % Fe₂O₃)along with between about 0.06 wt. % and about 3.6 wt. % K₂CO₃. Theseprecursor compounds may be mixed or blended in water or alcohol using,for example, a Cowles mixer, a ball mill, or a vibratory mill. Theseprecursors may also be blended in a dry form.

The blended mixture may then be dried if necessary in Step 104. Themixture may be dried in any of a number of ways, including, for example,pan drying or spray drying. The dried mixture may then be heated in Step106 at a temperature and for a period of time to promote calcination.For example, the temperature in the heating system used in heating Step106 may increase at a rate of between about 20° C. per hour and about200° C. per hour to achieve a soak temperature of about 1100° C.-1300°C., or about 1100° C. to 1250° C., which may be maintained for about twohours to about twelve hours. The heating system may be, for example, anoven or a kiln. The mixture may experience a loss of moisture, and/orreduction or oxidation of one or more components, and/or thedecomposition of carbonates and/or organic compounds which may bepresent. At least a portion of the mixture may form a hexaferrite solidsolution

The temperature ramp rate, the soak temperature, and the time for whichthe mixture is heated may be chosen depending on the requirements for aparticular application. For example, if small crystal grains are desiredin the material after heating, a faster temperature ramp, and/or lowersoak temperature, and/or shorter heating time may be selected as opposedto an application where larger crystal grains are desired. In addition,the use of different amounts and/or forms of precursor materials mayresult in different requirements for parameters such as temperature ramprate and soaking temperature and/or time to provide desiredcharacteristics to the post-heated mixture.

After heating, the mixture, which may have formed agglomerated particlesof hexaferrite solid solution, may be cooled to room temperature, or toany other temperature that would facilitate further processing. Thecooling rate of the heating system may be, for example, 80° C. per hour.In step 108, the agglomerated particles may be milled. Milling may takeplace in water, in alcohol, in a ball mill, a vibratory mill, or othermilling apparatus. In some embodiments, the milling is continued untilthe median particle diameter of the resulting powdered material is fromabout one to about four microns, although other particle sizes, forexample, from about one to about ten microns in diameter, may beacceptable in some applications. In a preferred embodiment, high energymilling is used to mill the particles to a fine particle size of 0.2 to0.9 microns in diameter. This particle size may be measured using, forexample, a sedigraph or a laser scattering technique. A target medianparticle size may be selected to provide sufficient surface area of theparticles to facilitate sintering in a later step. Particles with asmaller median diameter may be more reactive and more easily sinteredthan larger particles. In some methods, one or more alkali metals oralkali metal precursors or other dopant materials may be added at thispoint rather than, or in addition to, in step 102.

The powdered material may be dried if necessary in step 110 and thedried powder may be pressed into a desired shape using, for example, auniaxial press or an isostatic press in step 112. The pressure used topress the material may be, for example, up to 80,000 N/m, and istypically in the range of from about 20,000 N/m to about 60,000N/m.sup.2. A higher pressing pressure may result in a more densematerial subsequent to further heating than a lower pressing pressure.

In step 114, the pressed powdered material may be sintered to form asolid mass of doped hexaferrite. The solid mass of doped hexaferrite maybe sintered in a mold having the shape of a component desired to beformed from the doped hexaferrite. Sintering of the doped hexaferritemay be performed at a suitable or desired temperature and for a timeperiod sufficient to provide one or more desired characteristics, suchas, but not limited to, crystal grain size, level of impurities,compressibility, tensile strength, porosity, and in some cases, magneticpermeability. Preferably, the sintering conditions promote one or moredesired material characteristics without affecting, or at least withacceptable changes to other undesirable properties. For example, thesintering conditions may promote formation of the sintered dopedhexaferrite with little or minimal iron reduction. In one embodiment,the temperature used in the sintering step 114 is preferably between1100° C. to 1250° C. According to some embodiments, the temperature inthe heating system used in the sintering step 114 may be increased at arate of between about 20° C. per hour and about 200° C. per hour toachieve a soak temperature of about 1150° C.-1450° C. or about 1100° C.to 1150° C. or about 1100° C.-1250° C. which may be maintained for abouttwo hours to about twelve hours. The heating system may be, for example,an oven or a kiln. A slower ramp, and/or higher soak temperature, and/orlonger sintering time may result in a more dense sintered material thanmight be achieved using a faster temperature ramp, and/or lower soaktemperature, and/or shorter heating time. Increasing the density of thefinal sintered material by making adjustments, for example, to thesintering process can be performed to provide a material with a desiredmagnetic permeability, saturation magnetization, and/or magnetostrictioncoefficient. According to some embodiments of methods according to thepresent invention, the density range of the sintered hexaferrite may bebetween about 4.75 g/cm³ and about 5.36 g/cm³. A desired magneticpermeability of the doped hexaferrite may also be achieved by tailoringthe heat treatment of the material to produce grains with desired sizes.

The grain size of material produced by embodiments of the above methodmay vary from between about five micrometers and one millimeter indiameter depending upon the processing conditions, with even largergrain sizes possible in some aspects of methods according to the presentinvention. In some aspects, each crystal of the material may comprise asingle magnetic domain. Both doped CO₂Z and un-doped CO₂Z may be membersof the planar hexaferrite family called ferroxplana, having a Z-typeferrite crystal structure.

FIG. 3 illustrates a method 200 of forming textured Co₂Z according toanother embodiment adapted to reduce the magnetorestriction and improvethe resonant frequency of the material. The method 200 begins with step202 in which a fine grain hexagonal ferrite powder is formed. In oneimplementation, the fine grain hexagonal ferrite powder is a bariumcobalt ferrite Z-phase (Co₂Z) powder. The Co₂Z powder can be synthesizedusing a chemical process known in the art such as co-precipitation. TheCo₂Z can also be synthesized via sol-gel, calcining, and mechanicalmilling using a Netzsch zeta-mill or the like. In one embodiment, theCo₂Z powder has particle sizes of less than about 1 micron and surfaceareas of greater than about 6 m²/g. In another embodiment, the Co₂Zpowder has an average particle size of less than about 1 micron and anaverage surface area of greater than about 6 m²/g. In a preferredimplementation, the Co₂Z powder has a median particle size of between300-600 nm, and a surface area of greater than about 15 m²/g. It will beappreciated that the hexagonal ferrite powder can also comprise Y, W, U,X, or M phase hexagonal ferrite materials, depending on the application.

As FIG. 3 further shows, the method 200 further comprises step 204 inwhich the hexagonal ferrite powder is compacted by a known process suchas cold isostatic pressing, uniaxial pressing, extrusion, or the like.As also shown in FIG. 3, the hexagonal powder is subsequently fired at atemperature between about 1100° C. to 1250° C., which is lower than thestandard, conventional sintering temperature for the same material. Theresulting material is preferably a fine grained hexagonal ferritematerial.

FIGS. 4-6 illustrate impedance plots showing a Co₂Z powder having amedian particle size of about 2-3 microns and processed through azeta-mill and fired at about 1100° C. and 1140° C. As shown in FIG. 4,the resonant peak, or maximum of the imaginary permeability curve, isshifted to higher frequencies with zeta milling and low firingtemperatures. Without wishing to be bound by theory, it is believed thatthe hexagonal ferrite materials formed by the preferred processingtechniques do not have or have very small internal stress field leadingto magnetorestriction. The hexagonal ferrite material formed accordingto methods described herein can be incorporated in a variety of RFdevices such as high frequency antennas, inductors, and transformers.

FIGS. 4-6 show the change in the real component of the dimensionlesscomplex relative magnetic permeability, μ′ (referred to herein simply asthe magnetic permeability) versus frequency for CO₂Z powder. It can beseen that CO₂Z demonstrates a relatively constant magnetic permeability[at lower frequencies]. At higher frequencies, the material demonstratesa rise in magnetic permeability leading to a peak followed by a rapiddrop off as in magnetic permeability as frequency continues to increase.The peak of magnetic permeability will be referred to herein as the“resonant frequency.”

FIG. 4 depicts the magnetic permeability of CO₂Z powder with a medianparticle size of 2-3 microns. FIGS. 5 and 6 depict the magneticpermeability of the same CO₂Z powder that has additionally beenzeta-milled then fired at 1140° and 1100°, respectively. A comparison ofFIG. 4 with FIGS. 5 and 6 establishes that zeta-milling and firing theCO₂Z powder increases the resonant frequency of the material. Further, acomparison of FIGS. 5 and 6 shows that lowering the firing temperaturefrom 1140° to 1100° leads to a further increase in the resonantfrequency of the material. This increase in resonant frequency showsthat RF device components made from zeta-milled and low fired CO₂Z maybe capable of retaining their magnetic permeability and operating in afrequency range higher than, or in a broader frequency range than thatof similar devices or device components made from un-milled andhigher-fired CO₂Z.

FIGS. 4-6 also illustrate the effect of Zeta-milling and low firing onthe imaginary component of the complex relative magnetic permeability,μ″, which corresponds to energy loss in the material at highfrequencies. In FIGS. 4-6 it can be observed that maximum of theimaginary permeability curve, the “resonant peak” is shifted to higherfrequencies when the CO₂Z material is processed with powder that hasbeen zeta-milled and low fired.

FIG. 7 schematically shows an example of a circulator 300 incorporatinga enhanced resonant frequency Co₂Z material formed in accordance withcertain embodiments described herein. As shown in FIG. 7, the circulator300 has a pair of ferrite disks 802, 804 disposed between a pair ofcylindrical magnets 806, 808. The ferrite disks 802, 804 are preferablymade of a resonant frequency enhanced Co₂Z material according to certainembodiments of the present invention. The magnets 806, 808 can bearranged so as to yield generally axial field lines through the ferritedisks. Preferably, the ferrite disks have a magnetic resonance linewidthof 11 Oe or less.

FIG. 8 illustrates a telecommunication base station system 400comprising a transceiver 402, a synthesizer 404, an RX filter 406, a TXfilter 408, and magnetic isolators 410 and an antenna 412. The magneticisolators 410 can be incorporated in a single channel PA andconnectorized, integrated triplate or microstrip drop-in. In preferredimplementations, the magnetic isolators 410 comprise a Co₂Z materialmade in accordance with certain embodiments described in thisdisclosure.

Provided herein are various non-limiting examples of composition,materials, and methods of preparing the materials for electronicapplications. While the above detailed description has shown, described,and pointed out novel features of the invention as applied to variousembodiments, it will be understood that various omissions,substitutions, and changes in the form and details of the device orprocess illustrated may be made by those skilled in the art withoutdeparting from the spirit of the invention. As will be recognized, thepresent invention may be embodied within a form that does not provideall of the features and benefits set forth herein, as some features maybe used or practiced separated from others.

What is claimed is:
 1. A method of increasing the resonant frequency ofa hexagonal ferrite material, comprising: forming a fine grain hexagonalferrite powder comprising Z-phase barium cobalt ferrite having theformula Ba₃Co₂Fe₂₄O₄₁ and having an average particle size of between300-600 nm; and firing the hexagonal ferrite powder at a sinteringtemperature selected to form a hexagonal ferrite material having reducedmagnetorestriction.
 2. The method of claim 1 further comprisingcompacting the hexagonal ferrite powder prior to firing.
 3. The methodof claim 1 wherein the fine grain hexagonal ferrite powder has anaverage surface area of greater than 6 m²/g.
 4. The method of claim 1wherein the sintering temperature is between 1100 and 1250° C.
 5. Themethod of claim 3 wherein the fine grain hexagonal ferrite powder has anaverage surface area of greater than 15 m²/g.
 6. The method of claim 1wherein the fine grain hexagonal ferrite powder is formed by a processselected from the group consisting of co-precipitation, sol-gel, andmechanical milling.